1. Field of the Invention
The present invention relates in general methods and devices for producing or reforming a bellows, and more particularly methods and devices for producing or reforming a metal bellows installed in a flexible tube that is disposed in an exhaust pipe line of an automotive internal combustion engine to absorb undesired vibration of the exhaust pipe line and compensate relative displacement between two portions of the exhaust pipe line.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional flexible tube to which a bellows is practically applied will be briefly described with reference to FIGS. 5 and 6 of the accompanying drawings. The conventional flexible tube depicted by FIGS. 5 and 6 is described in detail in Japanese Utility Model First Provisional Publication 61-187916.
In FIG. 6, there is shown the flexible tube which is generally designated by numeral 100. As is seen from this drawing, the flexible tube 100 is a device arranged to connect upstream and downstream exhaust pipes xe2x80x9cUPxe2x80x9d and xe2x80x9cDPxe2x80x9d of an exhaust pipe line in such a manner as to absorb the vibration of the pipe line, and/or compensate a relative displacement between the two pipes xe2x80x9cUPxe2x80x9d and xe2x80x9cDPxe2x80x9d.
The flexible tube 100 comprises a metal bellows 102 that has an upstream end tightly disposed on a downstream end of the upstream exhaust pipe xe2x80x9cUPxe2x80x9d and a downstream end tightly disposed on an upstream end of the downstream exhaust pipe xe2x80x9cDPxe2x80x9d. A cover 104 of braided metal wire covers or encloses the bellows 102 having an upstream end tightly disposed on the upstream end of the bellows 102 and a downstream end tightly disposed on the downstream end of the bellows 102. For the tight mounting of the upstream and downstream ends of both the bellows 102 and the cover 104 onto the upstream and downstream exhaust pipes xe2x80x9cUPxe2x80x9d and xe2x80x9cDPxe2x80x9d, respective metal collars 106 and 108 are tightly disposed on the upstream and downstream ends of the cover 104, as shown. The bellows 102 can absorb vibration transmitted thereto from an internal combustion engine (not shown) through the upstream exhaust pipe xe2x80x9cUPxe2x80x9d. That is, upon receiving vibration, the bellows 102 is subjected to a certain resilient deformation due to the nature thereof, which absorbs the vibration and compensates a relative displacement between the upstream and downstream pipes xe2x80x9cUPxe2x80x9d and xe2x80x9cDPxe2x80x9d.
The cover 104 functions to restrict an excessive elongation of the bellows 102 and to protect the bellows 102 from being hit by small stones or the like flying from the road. That is, by a certain length, the cover 104, which is constructed of braided metal wire, can axially expand following the elongation of the bellows 102. Thus, when the elongation of the bellows 102 reaches to the certain length, the cover 104 now functions to stop the further elongation of the bellows 102. That is, due to provision of the cover 104, the bellows 102 can be protected from making an excessive elongation. In other words, the bellows 102 can expand axially by a certain length within the cover 104.
For assembling the flexible tube 100, the metal bellows 102 is reformed before being put into the cover 104. That is, the metal bellows 102 is subjected to a so-called xe2x80x9csingle compression processxe2x80x9d for achieving both a dimensional stability of the treated bellows 102 and an appropriate axial flexibility of the treated bellows 102. That is, as is seen from FIG. 5, in this compression process, a blank 102X of the bellows 102 is compressed once to the length xe2x80x9cLxe2x88x92axe2x80x9d which is shorter than the normal length xe2x80x9cLsxe2x80x9d of the bellows 102. This compression process is positively carried out for the reason originating from an inevitably occurring xe2x80x9cspring-back phenomenonxe2x80x9d of the compressed bellows 102Y. In fact, due to this spring-back phenomenon, after the compression, the over-compressed bellows 102Y gradually expands to have the normal length of xe2x80x9cLsxe2x80x9d. Furthermore, due to this compression, the pitch of bulges of the treated bellows 102 becomes small causing each bulge to have a generally xcfx89-shaped cross section, which brings about an appropriate axial flexibility or resilient deformation of the bellows 102.
However, it has been revealed that the above-mentioned single compression of the bellows 102X leaves in the bellows 102 a stress (or residual stress) of a type that causes the bellows 102 to expand in an axial direction when heated.
Accordingly, when the flexible tube 100 having the above-mentioned bellows 102 installed therein is practically used, that is, used in an exhaust pipe line of the engine, the entire length xe2x80x9cLsxe2x80x9d of the bellows 102 tends to increase due to releasement of the residual stress by the heat of the exhaust gas from the engine. The increase in the entire length xe2x80x9cLsxe2x80x9d of the bellows 102 however means a reduction in the certain length by which the bellows 102 can expand axially within the cover 104. That is, a so-called xe2x80x9celongation flexibilityxe2x80x9d of the bellows 102 is reduced or lowered at the time when the flexible tube 100 is being practically used.
The expansion/contraction of the bellows 102 is carried out while being interrupted by the cover 104. The cover 104 has such a structure as to reduce its diameter when axially expanded. Thus, elongation of the bellows 102 caused by application of the exhaust gas heat thereto brings about elongation of the cover 104 and thus reduces the diameter of the same. Reduction in diameter of the cover 104 narrows an annual space defined between the bellows 102 and the cover 104, which causes an obstacle to the elongation flexibility of the bellows 102. This fact will be understood from the graph of FIG. 4.
The graph of FIG. 4 shows a relationship between the force xe2x80x9cFxe2x80x9d needed for elongation of a bellows and the elongation xe2x80x9cExe2x80x9d of the bellows. In the graph, the solid line curve represents the elongation flexibility possessed by a normally dimensioned bellows 102A installed in the cover 104, which has the normal length xe2x80x9cLsxe2x80x9d and the critical elongation xe2x80x9cSxe2x80x9d. As is seen from this graphs in the normally dimensioned bellows 102A, within the region of the critical elongation xe2x80x9cSxe2x80x9d, the elongation xe2x80x9cExe2x80x9d of the bellows 102A increases substantially in proportional to the elongation force xe2x80x9cFxe2x80x9d applied to the bellows 102A. While, when the elongation xe2x80x9cExe2x80x9d extends beyond the critical elongation xe2x80x9cSxe2x80x9d, the elongation force xe2x80x9cFxe2x80x9d suddenly increases and thus the elongation flexibility of the bellows 102A is lowered. The broken line curve represents the elongation flexibility possessed by a bellows 102B somewhat expanded due to the exhaust gas heat applied thereto, which has the length xe2x80x9cLs1xe2x80x9d greater than xe2x80x9cLsxe2x80x9d and the critical elongation xe2x80x9cS1xe2x80x9d smaller than xe2x80x9cSxe2x80x9d. As is seen from the graph, in this bellows 102B, due to the reduction in critical elongation, the sudden increase of the elongation force xe2x80x9cFxe2x80x9d appears at an initial stage of the elongation xe2x80x9cExe2x80x9d. This means that the elongation flexibility of the bellows 102B is poor as compared with that of the bellows 102A. The phantom line curve represents the elongation flexibility possessed by a bellows 102C somewhat shorter than the bellows 102A due to excessive compression applied thereto, which has the length xe2x80x9cLs2xe2x80x9d smaller than xe2x80x9cLsxe2x80x9d and the critical elongation xe2x80x9cS2xe2x80x9d greater than xe2x80x9cSxe2x80x9d. Although this bellows 102C can provide a sufficient elongation under the practical use, the shorter initial length xe2x80x9cLS2xe2x80x9d of the bellows 102C brings about a difficulty with which the bellows 102C is installed in the cover 104. That is, in this case, the flexible tube 100 assembled fails to have a normally dimensioned structure.
According to tests executed by the inventors, the following facts have been further revealed. That is, when a bellows of the length of about 300 mm that has been subjected to a single compression process is left in a room temperature, the bellows is expanded or elongated by about 2 mm. While, when the bellows is practically used or heated by the exhaust gas from an engine, the bellows is expanded or elongated by about 6 to 8 mm. This means that even if the bellows is subjected to the single compression process, a certain stress (or residual stress) is left in the bellows, which causes the bellows to expand in an axial direction particularly when heated. The inventors have further revealed that the stress in the bellows can be sufficiently removed when the bellows is annealed at about 600xc2x0 C. for about 2 minutes. However, in this case, due to adding of the annealing process, the production process becomes complicated and thus the cost of the flexible tube is increased.
It is therefore an object of the present invention to provide a method of producing a bellows which is free of the above-mentioned drawbacks.
It is another object of the present invention to provide a producing device with which the method of the present invention can be practically carried out.
According to a first aspect of the present invention, there is provided a method for producing a metal bellows of a length of xe2x80x9cLsxe2x80x9d for use in a heated atmosphere. The method comprises the steps of (a) preparing a blank of the bellows, the blank being a metal pipe having a plurality of bulges formed therearound, the blank having a length xe2x80x9cLxe2x80x9d that is longer than the length xe2x80x9cLsxe2x80x9d; (b) axially compressing the blank by a length of xe2x80x9caxe2x80x9d, so that the compressed blank has a length of xe2x80x9cLxe2x88x92axe2x80x9d that is shorter than the length xe2x80x9cLsxe2x80x9d; (c) axially expanding the compressed blank so that the expanded blank has a length of xe2x80x9cLxe2x88x92a+bxe2x80x9d that is longer than the length xe2x80x9cLsxe2x80x9d; (d) removing the force that has been applied to the blank for expanding the same; and (e) leaving the treated blank in a room temperature until the time when the treated blank becomes to have the length of xe2x80x9cLsxe2x80x9d due to the spring-back phenomenon.
According to a second aspect of the present invention, there is provided a device for reforming a bellows for use in a heated atmosphere. The bellows is a metal pipe having a plurality of bulges formed therearound. The device comprises a base member; a first clamp device fixedly mounted on the base member, the first clamp device having a first hydraulically actuated clamping means which clamps one tubular end of the bellows when actuated; a second clamp device movably mounted on the base member, the second clamp device having a second hydraulically actuated clamping means which clamps the other tubular end of the bellows when actuated; a hydraulic power source which hydraulically actuates the first and second hydraulically actuated clamping means when assuming ON condition; and an electric moving device which moves the second clamp device toward and away from the first clamp device when electrically energized.